1. Technical Field of the Invention
The embodiments of the invention relate to communication devices and more particularly to a local oscillator feedthrough cancellation scheme to remove RF and baseband offsets.
2. Description of Related Art
Communication systems are known to support wireless and wire lined communications between wireless and/or wire lined communication devices. Such communication systems range from national and/or international cellular telephone systems to the Internet to point-to-point in-home wireless networks. Communication systems typically operate in accordance with one or more communication standards. For instance, wired communication systems may operate according to one or more versions of the Ethernet standard, the System Packet Interface (SPI) standard, or various other standards. Wireless communication systems may operate in accordance with one or more standards including, but not limited to, IEEE 802.11, Bluetooth, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), local multi-point distribution systems (LMDS), multi-channel-multi-point distribution systems (MMDS), and/or variations thereof.
Depending on the type of wireless communication system, a wireless communication device, such as a cellular telephone, two-way radio, personal digital assistant (PDA), personal computer (PC), laptop computer, home entertainment equipment, et cetera communicates directly or indirectly with other wireless communication devices. For direct communications (also known as point-to-point communications), the participating wireless communication devices tune their receivers and transmitters to the same channel or channels (e.g., one of the plurality of radio frequency (RF) carriers of the wireless communication system) and communicate over that channel(s). For indirect wireless communications, each wireless communication device communicates directly with an associated base station (e.g., for cellular services) and/or an associated access point (e.g., for an in-home or in-building wireless network) via an assigned channel. To complete a communication connection between the wireless communication devices, the associated base stations and/or associated access points communicate with each other directly, via a system controller, via the public switch telephone network, via the Internet, and/or via some other wide area network.
For each wireless communication device to participate in wireless communications, it includes a built-in radio transceiver (i.e., receiver and transmitter) or is coupled-to an associated radio transceiver (e.g., a station for in-home and/or in-building wireless communication networks, RF modem, etc.). Typically, the transceiver includes a data modulation stage and an RF stage. The data modulation stage (baseband process) converts between data and baseband signals in accordance with the particular wireless communication standard. The RF stage (transmitter section and receiver section) converts between baseband signals and RF signals. The RF stage may be a direct conversion transceiver that converts directly between baseband and RF or may include one or more intermediate frequency stages.
Wireless communication devices utilize gain control circuitry to provide control over amplification. One type of gain control circuitry incorporates differential amplifiers. For example, a Gilbert cell mixer may employ such differential amplification stage or stages where gain may be adjusted. Although there are variety of techniques to adjust gain, many amplifiers adjust the gain of the amplifier without adjusting the gain of the offset voltages. For example, with a Gilbert cell mixer, a transconductance stage is typically employed to convert a voltage input into current to drive the mixer stage. The transconductance stage generally generates some amount of offset, since not all of the transistors in the transconductance stage are matched. This offset may be represented as a small direct current (ΔDC) and when coupled to the mixer the ΔDC current causes a local oscillator feedthrough (LOFT) at the mixer output.
The offset may not be a significant concern, if the input signal to the transconductance stage is sufficiently higher than the offset, so that amplification of the offset is minimal compared to the actual signal. However, in other instances, the offset may be enough to affect the mixer drive current, which may result in appreciable LOFT. LOFT is a problem if it exceeds a leakage amount specified by a communication standard or if the feedthrough is sufficient to cause the receiver to not receive the signal properly.
Furthermore, current techniques do not sort out the LOFT contributions, so that two of the mechanisms that cause the LOFT at a transmit up-conversion stage are not separately cancelled. Baseband (BB) LOFT contribution generally originates from the device offsets in the digital-to-analog converter, low-pass filter, the transconductance stage(s) and any other baseband circuits in the signal path. The offsets mix with a local oscillator (LO) and generate a LO component at the output of the mixer. The second type, RF LOFT, is a direct coupling component either through parasitic capacitance or mutual inductance.
Accordingly, because there are at least two types of LOFT to address, it would be advantageous to implement a dual cancellation scheme to cancel both types of offsets separately at the up-conversion stage of a communication device.